My personal RPG gaming experiences are built upon the following pillars:
1) Combat Fun
No matter what game it is, I have to have fun killing stuff. This can be achieved through interesting animations, epic enemies, combos, group work, whatever. Considering the majority of my time in MMOs is spent in combat, this must be fun.
2) Sense of Achievement
I need to feel like I am taking on a challenge and beating it, because beating something I consider challenging makes me feel good. Again, this can come in many, many forms for different players. In most games, for me, this takes the form of beating hard bosses or beating hundreds of smaller mobs at once. In other games, this achievement has come from exploration, completion of deeds, acquisition of gear etc.
3) Completion of the Meta Game
Virtually all RPGs follow the same principles of combat: hit a button, perform a skill that does damage, eventually kill the enemy. The meta game is what I consider to be the specific mechanics that underpin the combat, from stat allocation, skill points, talents to gear, skills etc. Every game has a meta game and generally, for me, understanding the meta game and using it to my advantage is both fun and rewarding.
4) Constant Character Improvement
This can come from levels, gear acquisition, skill points, deed completion or whatever. I love "getting better" and it is something that drives me to continue playing RPGs long after I should have put the controller down! It fosters attachment to your character which again increases enjoyment of the game.
5) Enjoyment of the World
A well built world to explore is another key part of my RPG experience. Finding those hidden gems, those extra dungeons, that boss monster with no quest attached....a good game world is the setting for every single thing you do in game so it needs to be done right!
The reason I believe many modern MMOs fail to "wow" us any more is because our sense of achievement has been massively diminished and our enjoyment of the meta game has been destroyed by plagurism. Too many modern MMOs have "copied" the WoW formula. As a result, the learning curve is very short and easy which reduces our sense of achievement as a veteran MMO player. That, for me, leaves the combat, character advancement and the world as the main things left to enjoy, however in TOR specifically the character advancement is mostly pointless and the world is not well designed.
For example, I am currently playing Sacred 2 with my brother on the 360. The meta game is ridiculous, there is soooo much to think about when building your character that is has been enjoyable and extremely satisfying working out the meta game. Your character can reach level 200 so you are constantly leveling up which keeps you craving for more. There is also a ridiculous amount of loot so we literally leave no stone unturned because we might randomly find "the next weapon".
That no longer happens in MMOs because you know that by completing quests, you'll get rewards which are enough to level you up to max level. I have never once felt the need to loot a chest in TOR because i know it'll be mostly junk or just something equivalent to what i already have.
The solution, for new MMOs, is to be original. This means taking a risk which is why not too many people are doing it, but if well thought out then it will pay off. Give people a new meta game to learn, new mechanics to learn, new worlds to explore and make them feel good for doing it. You cannot give someone a sense of achievement if you dont make something hard for someone to achieve! To achieve the "wow" effect, you need to actually "wow" them and that is not possible when plagurising other people's games.